The effects of oxidizing conditions on vulcanizates obtained from polymers having carbon-carbon double bond unsaturation have long been a problem, particularly in applications where the vulcanizates are exposed to elevated temperatures for extended periods of time. A variety of approaches have been developed in the art in an attempt to solve this problem.
It is known that the carbon-carbon double bonds of such polymers activate the vulcanizate to oxidative attack. One solution to the problem of oxidative attack is to use polymers with few or no carbon-carbon double bonds. Examples of such polymers include butyl rubber (a copolymer of isobutylene and isoprene) which typically contain from 0.5 to 3.0 mole percent of carbon-carbon double bond unsaturation, and ethylene-propylene copolymers, which contain no such unsaturation.
Certain applications, such as the various hoses and seals in the engine compartment of automobiles, require vulcanized polymers with a combination of oil resistance and resistance to oxidative attack in air at elevated temperatures for extended periods of time. Vulcanizates of copolymers of conjugated dienes and α,β-unsaturated nitriles, such as acrylonitrile-butadiene copolymer, commonly known as nitrile rubber or NBR, are well known for their oil resistance. However, they contain carbon-carbon double bond unsaturation and therefore are susceptible to oxidative attack unless subjected to special compounding procedures for the production of oxidation resistant vulcanizates.
In order to reduce the amount of carbon-carbon double bond unsaturation in NBR and yet retain the copolymer's oil resistance, which is thought to be provided by the nitrile functional groups in the copolymer, methods have been developed to selectively hydrogenate the carbon-carbon double bond unsaturation of NBR without hydrogenating the nitrile groups to produce hydrogenated NBR or HNBR. See for example, British patent 1,558,491, the contents of which are hereby incorporated by reference.
While the development of HNBR has been a significant advance in the art, there is still room for improvement. Specifically, there is a continuing need to develop nitrile polymer vulcanizates, which have improved physical properties such as hot air aging, and the like.